1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to a method for fabricating a semiconductor device, in which a grain size is made coarse for forming a thin film with a low resistance.
2. Discussion of the Related Art
As semiconductor device packing advances, widths of lines therein are in general reduced, with increased resistance that causes problems, such as slowing down a device operation speed. To solve the problem of an increased sheet resistivity, a thickness of the line may be increased while reducing a width of the line, which again causes problems of complicated fabrication process and low yield due to difficulty in gap filling in deposition of an interlayer insulating film between the lines having a greater aspect ratio, with a high possibility of void formation. In order to solve these problems, a refractory metal silicide, such as tungsten silicide Wsix, titanium silicide TiSix, or cobalt silicide CoSix or the like is formed on a polysilicon layer in a background art, for preventing increased resistivity. Even though the formation of the refractory metal silicide may improve a resistivity and a step coverage to some extent, an improved method for forming a polycide has been in need.
A background art method for fabricating a semiconductor device will be explained with reference to the attached drawings. FIGS. 1a.about.1c illustrate sections showing the steps of a background art method for fabricating a semiconductor device.
Referring to FIG. 1a, the background art method for fabricating a semiconductor device starts with forming a silicon oxide film SiO.sub.2 12 on a semiconductor substrate 11, for use as a gate insulating film. Then, a polysilicon layer 13, a tungsten nitride film 14, and a pure tungsten film 15 are formed on the silicon nitride film 12 in succession, for use as a gate electrode. The tungsten nitride film 14 is formed very thin, and the tungsten film 15 is reactive sputtered. The tungsten nitride film 14 is formed for preventing reaction between the tungsten film 15 and the polysilicon layer 13, that forms tungsten silicide at an interface of the tungsten film 15 and the polysilicon layer 13. The tungsten silicide at the interface of the tungsten film 15 and the polysilicon layer 13 increases a sheet resistivity. Then, a photoresist film 16 is coated on the tungsten film 15, subjected to exposure and development, to pattern the photoresist film 16 to define a gate region. As shown in FIG. 1b, the patterned photoresist film 16 is used as a mask in selectively removing the tungsten film 15, the tungsten nitride film 14, the polysilicon layer 13, and the silicon oxide film 12, to form a gate electrode 18 and a gate insulating film 12a. As shown in FIG. 1c the photoresist film 16 is removed, the gate electrode 18 is used as a mask in lightly doping the semiconductor substrate 11, and insulating sidewalls 19 are formed at both sides of the gate electrode 18. The gate electrode 18 and the insulating sidewalls 19 are used as a mask in doping the semiconductor substrate 11 heavily, to form source/drain impurity regions 17 in surfaces of the semiconductor substrate 11 on both sides of the gate electrode 18.
However, the background art method for fabricating a semiconductor device has the following problems.
First, the additional tungsten thin film formation process makes the fabrication process complicated, with a reduction of productivity.
Second, formation of an even and thin, to a few tens of .ANG., tungsten nitride film is difficult.
Third, the low temperature approx. 300.degree. C. in formation of the tungsten thin film hampers formation of the tungsten thin film with coarse grains because diffusion of tungsten atoms should be accompanied for growth of crystal grains in the tungsten thin film.